Idealized voltammogram

The four values obtained from the voltammogram provide three important insights that are readily visible in the idealized voltammogram ... 

In many situations, such readily interpretable ideal or near-ideal voltammograms are not obtained. There are numerous factors that can distort the voltammogram from this ideal shape, of which three will be briefly and qualitatively discussed here chemical irreversibility, electrochemical irreversibility, and solution resistance iR drop. Textbooks have an in-depth coverage of these issues. 

In the ideal case, reversible cyclic voltammograms of redoxactive films should show completely symmetrical and mirror-image cathodic and anodic waves with identical peak potentials and current levels 34-i37) pjg... 

LCEC is a special case of hydrodynamic chronoamperometry (measuring current as a function of time at a fixed electrode potential in a flowing or stirred solution). In order to fully understand the operation of electrochemical detectors, it is necessary to also appreciate hydrodynamic voltammetry. Hydrodynamic voltammetry, from which amperometry is derived, is a steady-state technique in which the electrode potential is scanned while the solution is stirred and the current is plotted as a function of the potential. Idealized hydrodynamic voltammograms (HDVs) for the case of electrolyte solution (mobile phase) alone and with an oxidizable species added are shown in Fig. 9. The HDV of a compound begins at a potential where the compound is not electroactive and therefore no faradaic current occurs, goes through a region... 

In an ideal case the electroactive mediator is attached in a monolayer coverage to a flat surface. The immobilized redox couple shows a significantly different electrochemical behaviour in comparison with that transported to the electrode by diffusion from the electrolyte. For instance, the reversible charge transfer reaction of an immobilized mediator is characterized by a symmetrical cyclic voltammogram ( pc - Epa = 0 jpa = —jpc= /p ) depicted in Fig. 5.31. The peak current density, p, is directly proportional to the potential sweep rate, v ... 

FIGURE 1.11. Convolution of the cyclic voltammetric current with the function I j Jnt, characteristic of transient linear and semi-infinite diffusion. Application to the correction of ohmic drop, a —, Nernstian voltammogram distorted by ohmic drop , ideal Nernstian voltammogram. b Convoluted current vs. the applied potential, E. c Correction of the potential scale, d Logarithmic analysis. 

Herrero and Abruna [25] have also studied the kinetics and mechanism of Hg UPD on Au(lll) electrodes in the presence and absence of bisulfate, chloride, and acetate ions. In the absence of the interacting anions (in perchloric acid), the Hg UPD was significantly controlled by gold-mercury surface interactions. In sulfuric acid solutions, the kinetics of the initial and final stages of mercury deposi-tion/dissolution was altered. The presence of two well-ordered structures at potentials below and above mercury deposition led to the formation of two pairs of sharp spikes in cyclic voltammograms. In the chloride medium, the voltammetric profile exhibited two sharp peaks and thus it was very similar to that obtained in sulfuric acid solution. Neither nucleation, nor growth kinetics mechanism was found to be linked to the process of formation/disruption of the mercury chloride adlayer. The transients obviously deviated from the ideal Langmuir behavior. 

A plot of i versus E (Eq. 3.42) defines the ideal thin-layer voltammogram shown in Figure 3.33. The contrast with voltammetry under quasi-infinite conditions should be evident. Note that the peak is symmetrical about E° and that the current drops to zero after the potential passes through Ep, and that ip is directly proportional to scan rate. The number of coulombs passed in fully traversing the peak is given by Equation 3.36 since the conversion of R to O is complete. 

The diffusion of the electroactive ions is both physical and due to electron transfer reactions.45 The occurrence of either or both mechanisms is a function of the electroactive species present. It has been observed that the detailed electrochemical behaviour of the electroactive species often deviates from the ideal thin film behaviour. For example, for an ideal nemstian reaction under Langmuir isotherm conditions there should be no splitting between the anodic and cathodic peaks in the cyclic voltammogram further, for a one-electron charge at 25 °C the width at half peak height should be 90.6 mV.4 In practice a difference between anodic and cathodic potentials may be finite even at slow scan rates. This arises from kinetic effects of phase formation and of interconversion between different forms of the polymer-confined electroactive molecules with different standard potentials.46... 

Under equilibrium conditions, the values of ped, ped — , d and i 1 are independent of the kinetic parameters k°, a and v. Ideally, the part of the voltammogram recorded during the forward sweep (here the reduction wave) satisfies the following three criteria, where the numerical values given in units of mV refer to T = 298 K. 

Scheme I. Concept of a two-terminal microsensor showing two possible idealized responses to a species L which binds to the indicator molecule M. A) The linear sweep voltammograms reveal a difference between the current peaks for oxidizing the reference molecule, R, and M or M-L. The position of the current peak along the potential axis, V, is variable and depends on the concentration of L. B) The linear sweep voltammograms reveal a decrease in amplitude for the current peak assigned to M and proportional growth of a new current peak assigned to M-L.

Figure 5 represents an ideal reversible one-electron transfer process in the absence of drop or capacitative charging current, although in real experiments contributions to the response from both these terms are unavoidable. Figure 6 shows the effect of uncompensated resistance for both transient and steady-state voltammograms, whilst Fig. 7 shows the influence of double layer capacitance on a cyclic voltammetric wave. Note that for steady-state voltammetric techniques only very low capacitative charging... 

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